Mount Prospect, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

699
FXUS63 KLOT 170753 CCA
AFDLOT

Area Forecast Discussion...CORRECTED
National Weather Service Chicago/Romeoville, IL
253 AM CDT Wed Jun 17 2026

.KEY MESSAGES...

- An outbreak of severe thunderstorms with the potential to
  produce damaging tornadoes, destructive straight line winds,
  and large hail is possible this afternoon into the early
  evening.

- A flood watch has been issued for today. Locally heavy
  rainfall is expected to result in an enhanced threat of
  localized flash flooding.

- Tranquil weather is expected to close out the work week in the
  wake of this system.

- Another strong storm system is expected to bring a return of
  showers and thunderstorms later this weekend, with some
  potential for heavy rainfall and/or severe weather somewhere
  in the general region.

&&

.DISCUSSION...
Issued at 212 AM CDT Wed Jun 17 2026

Through this evening:

Complex set-up today with difficult to predict mesoscale
details regarding early day convection and its impacts on
surrounding environment likely to play a key role in the
magnitude and location of severe potential this afternoon and
evening. Significant regional severe weather outbreaks are
often a result of mesoscale "accidents" happening withing an
otherwise favorable synoptic scale environment for severe
weather.

Synoptically, today fits that bill well with an unseasonably
strong 120kt+ 250mb upper level jet nosing east-southeast into
the mid-Mississippi Valley this afternoon. A rather sharp and
vigorous 500mb trough will reach the Upper Mississippi Valley
later this afternoon, with a remarkably strong (for mid June)
80kt+ H5 jet streak rounding the base of this trough. At the
sfc, a near record deep (for June) double barreled low pressure
system is progged by 00z to have one low over southern Lake
Michigan east of the IL/WI border with a trough extending
northwest to a second low over northwest Wisconsin. From a large
scale perspective, this set-up looks exceptionally favorable
for a regional severe weather outbreak including the potential
for intense tornadoes, however the devil may end up being in the
mesoscale details that unfortunately just cannot be accurately
predicted this far in advance.

A strong low level jet over the eastern Plains is rapidly
transporting deep moisture northward into the lower Missouri
Valley. This process can be seen well on GOES TPW imagery. The
combination of strong large scale ascent associated with the
upper level jet and strong isentropic ascent with the
aforementioned low level jet has already led to the development
of showers and scattered thunderstorm from northern IA west-
northwest across South Dakota and southern Minnesota. Convective
allowing models have a pretty strong signal depicting more
vigorous, potentially severe, convection developing over eastern
IA just prior to dawn this morning as this strong forcing
encounters a rapidly northeastward expanding axis of elevated
instability driven by the strong low level theta-e advection
from the low level jet. The severe threat with this convection
looks like it will be confined more to areas south and west of
our CWA, closer to the expected elevated instability axis, but
cannot rule out a severe threat later this morning creeping
into our far southwestern CWA.

There is considerable spread in guidance in how this convection
evolves, but with low level jet remaining strong through the day
as the nose shifts east into Illinois, it is certainly plausible
that this convection will increase in coverage and organize into
an MCS. Depending on the track, longevity, and intensity of the
potential MCS, the synoptic scale environment will be disrupted
heading into the afternoon. Again, there`s considerable spread
in guidance with respect to the placement of what will likely
become the effective boundary, a composite of the synoptic scale
warm front and outflow boundary from this MCS. A number of
models indicate an extremely rapid northward surge of low level
moisture and result instability in the wake of this MCS, while
other guidance indicates that the effective boundary may
struggle to even reach the Highway 24 corridor across our far
southern CWA.

Normally it is safe to expect a farther south than progged
location of the effective boundary in the wake of an early
afternoon MCS, however today`s set-up features an unseasonably
deep surface low for mid June with a near peak high sun angle.
These factor make it difficult to write off the models that
depict this boundary and unstable air mass surging rapidly north
in the wake of the MCS, possibly making it to nearly the
Wisconsin border later this afternoon. The northward extent of
the boundary should be demarcation between a more substantial
severe weather and tornado threat, and a potentially more muted
(but still existent) severe weather threat to the north.

Several of the CAMs are depicting convective initiation along a
pre-cold front trough in the warm sector. Given the expected
shear, should air mass recovery actually be sufficient, then
this would create an exceptionally favorable environment for
discrete, fast moving tornadic supercells. Given how quickly
these cells will tend to propagate off of the initiating
boundary, their ability to survive and thrive will hinge on them
traversing a sufficiently unstable/weakly capped environment.
Where and whether this type of environment will exist, will
hinge on how the difficult to predict mesoscale details play out
this afternoon. Also, the magnitude of instability will play a
role in how quickly these storm develop along the boundary as
low level convergence along the prefrontal trough isn`t progged
to be terribly strong. So if weak convection quickly moves off
the boundary to even a weakly capped air mass, it could struggle
to mature. Where and whether this type of environment will
exist, will hinge on how the difficult to predict mesoscale
details play out this afternoon.

Forecast hodographs within the warm sector feature an amazingly
strong low level shear profile with 0-1km SRH well over 300
m^2/s^2, maximized closest to the composite warm front/outflow
boundary where some backing of the low level flow is likely.
Given that LCLs and LFCs will be quite low, there could be
rather strong low level CAPE despite the progged abysmal low
level lapse rates (less than 6C/km).

Ultimately, today is going to require close monitoring of the
mesoscale environment to determine if and where the greatest
tornado threat will exist this afternoon and early evening. The
current SWODY1 placement of the MDT risk highlights the most
likely area within our CWA to face this potential, however it
also wouldn`t be hard to envision a scenario where early
convection results in the more favorable environment developing
farther south/east than the current MDT risk. Given these
uncertainties, not planning any significant change to our
messaging for the severe potential at this time, however as
observational trends begin to shed a light on the ultimate
evolution of the mesoscale environment, some changes to today`s
severe whether forecast may be needed.

Finally, the influx of richer Gulf moisture combined with the
strong synoptic forcing continues to look favorable for locally
heavy rainfall totals today. While storm motion will be fast,
rainfall efficiency will be high, so localized totals in excess
of 3" (most of it falling in a very short time) are certainly
well within the realm of possibility. Flash flood guidance
remains low, so in coordination with neighboring offices will be
issuing a Flood Watch for today.

Beyond this evening:

Gusty westerly winds are expected overnight as the deep low
pulls away. Dry and pleasant weather is expected Thursday and
Friday. Another strong shortwave trough is progged to move
across the Midwest later this weekend, likely resulting in
another round of potentially heavy rainfall and/or severe
weather in the general region. Too soon to say how much, if any
of our CWA will be impacted, but our area is certainly well
within the proverbial cone of uncertainty and pops are very high
Sunday accordingly.

- Izzi

&&

.AVIATION /06Z TAFS THROUGH 12Z THURSDAY/...
Issued at 1250 AM CDT Wed Jun 17 2026

Forecast concerns include...

Scattered thunderstorms from mid morning through early evening.
Ifr cigs late morning through late afternoon.
Wind directions mid/late afternoon.

The primary focus for this period is thunderstorm timing. Models
are in general agreement with a thunderstorm complex moving
southeast from eastern IA around daybreak through north central
IL in the late morning/early afternoon, with showers and
embedded thunder on the north side of this complex. Prob thunder
for this potential seems reasonable at this time but did
include a few hours of mvfr vis/cigs with moderate rain. There
may not be much of a break/lull between this first wave and
additional thunderstorm development in the early/mid afternoon.
Previous forecast tempo from 19z-23z seems reasonable and there
is some potential for thunder to be occurring during most of
this time period at the Chicago terminals. The thunderstorms
are expected to move southeast of the terminals by early this
evening with a few lingering showers possible into this evening.
Any of the thunderstorms today will be capable of producing
torrential rainfall which could briefly lower visibilities under
1sm.

Cigs are expected to lower through mvfr during the mid/late
morning and into ifr by early afternoon, which may continue
through late afternoon before lifting and scattering out by
early this evening.

Light southwest winds overnight will become southerly by
daybreak and then southeasterly during the mid/late morning.
Speeds and gusts will increase with gusts potentially into the
mid 20kt range by late morning/early afternoon. Then there is
quite a bit of uncertainty for wind directions during the
mid/late afternoon. Some of the guidance lifts the warm front as
far north as ORD, which would shift winds back to the south/
southwest. For now, maintained southeast winds at ORD and MDW
and turned winds southwest at GYY. Changes to the wind forecast
during this time period can be expected with later forecasts.
Winds will become west/southwest by early evening and then more
westerly early Thursday morning as speeds/gusts increase. cms

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Flood Watch from 10 AM CDT this morning through this afternoon
     for ILZ003-ILZ004-ILZ005-ILZ008-ILZ010-ILZ011.

     Flood Watch from 10 AM CDT this morning through this evening
     for ILZ006-ILZ012-ILZ013-ILZ019-ILZ020-ILZ021-ILZ023-
     ILZ032-ILZ033-ILZ039-ILZ103-ILZ104-ILZ105-ILZ106-ILZ107-
     ILZ108.

     Beach Hazards Statement from this afternoon through Thursday
     morning for ILZ006-ILZ103-ILZ104.

IN...Flood Watch from 10 AM CDT /11 AM EDT/ this morning through
     this evening for INZ001-INZ002-INZ010-INZ011-INZ019.

LM...Small Craft Advisory from 1 PM this afternoon to 1 AM CDT
     Thursday for the IL nearshore waters.

     Small Craft Advisory from 1 PM this afternoon to 7 PM CDT this
     evening for the IN nearshore waters.

&&

$$

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